The present invention relates to a developing device for developing an electrostatic latent image formed on an image supporter by means of a developing agent and an image forming device including the developing device, and particularly to a developing device in which the developing agent is carried using an electric field in which the distribution of electric potentials varies over time, in the manner of traveling waves having a predetermined direction of movement (hereinafter, the foregoing electric field will be simply referred to as a traveling wave electric field).
As a developing device used in image forming devices using an electronic photo process such as photocopiers and laser printers, what have particularly drawn attention are developing devices which carry out developing without causing an image supporter such as a photosensitive drum to contact with a developing agent carrier such as a developing sleeve. As such kinds of developing devices, for instance, developing devices of a powder cloud type, of a jumping development type, and of a traveling wave electric field type have been proposed.
As a developing device using a traveling wave electric field, Japanese Laid-Open Patent Application No. 59-181371/1984 (Tokukaisho 59-181371; published on Oct. 15, 1984) and Japanese Laid-Open Patent Application No. 59-189371/1984 (Tokukaisho 59-189371; published on Oct. 26, 1984) disclose respective developing devices.
These developing devices disclosed by the respective publications are both arranged in such a manner that a power source for generating alternating voltages each having a different phase and a plurality of electrodes provided on a plate-shaped developing agent carrying member at predetermined intervals are provided, and a developing agent is supplied to a photosensitive drum by means of a traveling wave electric field generated by applying the alternating voltages to the electrodes.
However, it is noted that these conventional arts have such problems as the generation of an electric charge on the developing agent carrying member and the adhesion of the developing agent to the developing agent carrying member.
When the developing agent carrying member is charged, the electric charge on the developing agent carrying member could disrupt the traveling wave electric field so as to have an adverse impact on the condition of the carrying of the developing agent, or could vary a developing electric field in the process of developing on the photosensitive drum so that the density of a developed image is varied. Also, the developing agent adhered on the developing agent carrying member could cause an adverse effect on the process of carrying the developing agent so that the irregularity of density is generated in a developed image.
To solve these problems, the inventors of the present invention have thought that a developing device including: a looped belt member for covering the developing agent carrying member; and a drive roller provided so as to be in touch with the inner surface of the belt member, so that (i) the discharge of the surface of the developing agent carrying member, the surface being used for carrying the developing agent, and (ii) the removal of the residual developing agent, are carried out by rotating the drive roller so as to move the belt member (Japanese Patent Application 2000-283087 (filed on Sep. 19, 2000)).
With the developing device of the foregoing patent application, it is possible to restrain the generation of the electric charge on the developing agent carrying member and the degradation of image quality due to the irregularity of density.
Now, if the developing agent carrying member is irregularly in touch with the belt member, the traveling wave electric field is irregularly formed on the surface of the belt member and this could hinder the developing agent from being stably carried. When the developing agent is not stably carried, the irregularity of density occurs in a developed image. For this reason, it is necessary to bring the developing agent carrying member into contact with the belt member evenly and almost absolutely.
However, provided that the developing agent carrying member is brought into contact with the belt member evenly and almost absolutely, the frictional force between the developing agent carrying member and the belt member causes the rotating belt member to be under load conditions. Thus, in the case of the developing device of the foregoing Patent Application, it is necessary to rotate the belt member by the drive roller in order to cope with the load associated with the friction force.
Thus, as in the following description, there are three methods of rotating the belt member.
The first method is to increase the driving force by increasing the tension of the belt member. However, this brings about such a problem that increasing the tension of the belt member increases the load due to the frictional force between the developing agent carrying member and the belt member.
The second method is to decrease the tension of the belt member so as to directly reduce the load caused by the frictional force between the developing agent carrying member and the belt member. However, when the tension of the belt member is decreased, a slight gap is produced between the developing agent carrying member and the belt member so that the belt member vibrates because of the traveling wave electric field formed on the developing agent carrying member, and hence unpleasant noise is made.
The third method is to increase the size of the drive roller in order to enlarge the contact area between the drive roller and the belt member. However, this increases the size of the developing device and also drives up the costs.
In summary, these three methods share such a common problem that it is difficult to realize an arrangement for rotating the belt member by the drive roller while keeping the developing agent carrying member to be in good contact with the belt member.
The present invention has been done in view of the foregoing problems, so that the object of the present invention is to offer a developing device which is quiet, small in size, and manufactured at low cost, and an image forming device including the developing device.
To solve the foregoing problems, a developing device in accordance with the present invention is characterized by comprising: developing agent carrying means for carrying a developing agent towards an image supporter, using an electric field in which distribution of electric potentials varies in a manner of traveling waves having a predetermined direction of movement; a belt member which is provided so as to cover a surface of the developing agent carrying means, the surface facing the image supporter; drive means which is in touch with the belt member, for rotating the belt member around the developing agent carrying means; and drive assistance means which is provided so as to sandwich the belt member with the drive means.
That is to say, the developing device of the present invention is arranged in such a manner that the developing agent carrying means carries the developing agent towards the image supporter using the electric field (traveling wave electric field) in which distribution of electric potentials varies in a manner of traveling waves having a predetermined direction of movement. In the developing device in accordance with the present invention, the surface of the developing agent carrying means could be charged due to the contact with the developing agent, and the developing agent could adhere to the developing agent carrying means. When such problems as the charging of the developing agent carrying means occur, there may be cases where the developing agent is not stably carried and the density of an image obtained by developing an electrostatic latent image is irregular.
Thus, the developing device of the present invention comprises the belt member provided so as to cover the surface of the developing agent carrying means, the surface facing the image supporter, and the drive means which is in touch with the belt member so as to rotate the belt member around the developing agent carrying member.
Rotating the belt member by the drive means makes it possible to cause the surface of the belt member, which is not charged or to which no developing agent adheres, to face the image supporter. With this arrangement, it is possible to stably carry the developing agent and to develop an image with little irregularity of density.
When the developing agent carrying means is irregularly in touch with the belt member, the traveling wave electric field appearing on the surface of the belt member, which is extended from the developing agent carrying means via the belt member, is also irregular so that it could be impossible to carry the developing agent stably.
In the meantime, when the developing agent carrying means is uniformly in touch with the belt member, the frictional force between the developing agent carrying member and the belt member causes the rotating belt member to be under load conditions.
For this reason, the developing device in accordance with the present invention particularly includes the drive assistance means provided so as to sandwich the belt member with the drive means. In other words, the belt means is sandwiched between the drive means and the drive assistance means. With this arrangement, the contact pressure between the drive means and the belt member is increased so that it is possible to rotate the belt member with the force stronger than the load caused by the frictional force between the developing agent carrying means and the belt member.
Moreover, since the driving force is enhanced by increasing the contact pressure between the drive means and the belt member, the drive means can rotate the belt member even if the belt member is at load conditions to some degree. Therefore, since it is unnecessary to lower the tension of the belt member in order to decrease the load, the generation of unpleasant noise caused by the vibration of the belt member can be prevented and hence the quietness of the device can be improved.
Moreover, in the foregoing arrangement, since the driving force of the drive means can be enhanced by increasing the force of sandwiching the belt member between the drive means and the drive assistance means, it is unnecessary to increase the size of the drive means in order to increase the contact area between the drive means and the belt member. Thus, it is possible to acquire sufficient driving force using small drive means, so that the developing device can be downsized and the costs thereof can be reduced.
For a fuller understanding of the nature of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawing.